Technical Field
The present invention relates to an image forming apparatus such as a copier, a facsimile machine, a printer, or a multi-function apparatus having one or more capabilities of those devices.
Related Art
Demand for faster, more energy-efficient image forming apparatuses such as printers, copiers, and facsimile machines is acute. In these types of image forming apparatuses, an unfixed toner image is formed on a recording medium via image forming processes employing an image transfer method or a direct method. The recording medium may include a recording sheet, a printed sheet, a photosensitive sheet, an electrostatic recording sheet, and the like. Similarly, the image forming processes include electrophotographic recording, electrostatic recording, magnetic recording, and the like.
As a fixing device to fix an unfixed toner image, various types of fixing devices are used including a contact heating method from a heat-roller method, a film- or belt-heating method, and an electro-magnetic induction heating method.
Using such a thin fixing belt with a low thermal capacity can drastically reduce the amount of energy necessary for heating the fixing belt, and warm-up time or a first-print time can be reduced. Herein, “warm-up time” means a length of time that the fixing device takes to heat up from room temperature to a predetermined printing temperature when the power is turn on; i.e., a reload temperature capable of performing printing. “First-print time” means the time it takes from receipt of a print request and preparation for printing to completion of a printing operation and sheet discharge. The first-print time is also requested to be short. In addition, in accordance with the higher printing speed of the image forming apparatus, the number of prints per unit time increases and a required heat quantity drastically increases. In particular, upon the start of continuous printing, thermal capacity is inadequate and a so-called temperature drop occurs, which is a problem.
The warm-up time can be shortened with so-called SUrface Rapid Fixing (SURF) technology using a ceramic heater, by which a compact and efficient fixing device is realized compared to the belt configuration. However, SURF locally heats a nip portion alone and other parts are not heated. Accordingly, the belt is coolest at an inlet to the nip portion and defective fixing may occur as a result. In high-speed apparatuses in which the belt rotates fast and heat is discharged from the belt at portions other than the nip, defective fixing tends to occur more frequently.
To solve these problems, for example, JP-2007-334205-A proposes a structure using an endless belt, in which the whole belt can be heated, the first-print time from the heating standby time can be reduced, and the thermal capacity deficiency during high-speed printing is remedied, thereby obtaining optimal fixability even though mounted in a high-performance apparatus.
FIG. 1 shows an overview of the fixing device of JP-2007-334205-A. A metallic, thermally conductive pipe 2 disposed in an interior of an endless belt 1 as a fixing belt, is fixed to the fixing device so as to guide moving of the endless belt 1. A heat source 3 inside the conductive pipe 2 heats the endless belt 1 via the conductive pipe 2. The fixing device further includes a pressure roller 4 including a metal roller 4a and an elastic layer 4b and contacting the conductive pipe 2 via the endless belt 1 to thus form a nip N. The endless belt 1 moves cyclically together with the rotation of the pressure roller 4. With this structure, the entire endless belt constructing the fixing device can be heated, the first-print time from the heating standby time can be shortened, and the heat shortage in the high-speed printing can be eliminated.
Use of a structure to directly heat the endless belt enables to further save the energy and shorten the first-print time from the heating standby time. However, in a fixing device with a low thermal capacity, when sheets with a size different from the size set by the image forming apparatus is conveyed, the fixing device is not prevented from being heated excessively, thereby damaging the fixing member.
The above-described malfunction occurs due to inconsistent fixing control. Namely, although the fixing control should be performed based on the actual sheet conveyance condition, the image forming apparatus instructs the fixing device to operate under the conditions set for a different sheet size. As a result, temperature rise occurring at untargeted end portions cannot be prevented, and thus, the fixing device is heated exceeding its upper temperature limit and is eventually damaged. In such a case, the fixing control should be changed so that the end temperature rise does not occur. However, in the conventional method, the temperature rise is not detected and prevented.
More specifically, even though the image forming apparatus can set the sheet size for the sheets for fixing to be performed by the heat source disposed in the center and another heat source disposed nearer to the end than the center, because in actuality, the sheet does not pass through the heating area to be heated by the heat source disposed nearer to the end, the size of the sheet that passes through only the heating area heated by the inner heat source is set. As a result, the fixing device is damaged.